1. Field of the Invention
This invention relates to driving capacitive loads and, more particularly, to driving liquid crystal displays (“LCDs”).
2. Description of Related Art
LCDs are in widespread use today, and their popularity is expected to increase. These devices operate by controlling the amount of light that is passed or reflected by a set of liquid crystal (LC) elements arranged in rows and columns in the display. Each LC element comprises a pair of plates surrounding liquid crystal material. The amount of light that is passed or reflected by an LC element is controlled by the voltage that is delivered to the plates of that element.
To maintain the amount of light passed or reflected by the LC element at a constant level, the voltage across the element must usually be reversed in polarity periodically. As a result, an AC signal is typically used to drive the element, the magnitude of the signal determining the amount of light that is passed or reflected.
A typical LCD has hundreds of thousands of LC elements arranged in hundreds of rows and columns. To reduce the amount of circuitry that is needed to drive each LC element, all LC elements in the same row typically communicate through a single row line, while all elements in the same column typically communicate through a single column line. Each LC element is thus uniquely defined by the row and column line that intersect at its location. The voltage across each element is regulated by controlling the amount of charge that is delivered to it through its coordinating row or column line.
The picture displayed by an LCD is typically painted by sequentially scanning each line of the display, somewhat like the way a picture is painted in a television set. For example, the first row line might be activated, followed by the delivery of the desired signal to the first column line, thus establishing the desired voltage across the first element in the first row. While the first row line is still activated, the desired signal would then be delivered to the second column line, thus establishing the desired voltage across the second element in the first row. This process would typically continue until all of the elements in the first row are set to their desired levels. Alternatively, the desired voltage across all of the elements in a row can be applied at the same time.
The second row line would then be activated, followed by the sequential or simultaneous charging of each LC element in the second row. This process would continue until the voltages across all of the LC elements in the display are set to their desired levels. This entire cycle would then repeat itself a short time later, but with the voltages being of opposite polarity, to provide the refreshment needed for each LC element.
Electronic switches are often used to controllably connect and disconnect each element to its associated column line. The control input to these switches is typically connected to the row line at which each switch resides. These switches, however, also often have intrinsic capacitance.
Although only one LC element in a column is typically charged at a time, the switches that are associated with the elements that are not being driven typically also impose a significant amount of capacitance on the column line through which the voltage is being delivered to the single element that is being driven. Because there are typically hundreds of rows of LC elements that are connected to the column line through which the voltage is being delivered to the single element, the combined effect of the capacitance imposed by these inactive switches often imposes hundreds of times the amount of capacitance that is exhibited by the single element that is being driven.
There is also often significant intrinsic capacitance between the lines that control the LC elements and the backplane of the display.
This very large amount of combined capacitance on the column lines often causes large amounts of energy to be dissipated during the use of the LCD. As the voltage on each LC element is being reversed in polarity, the voltage on the much-larger capacitance that is imposed on the line must also usually be changed. This typically requires a large amount of current. In turn, the passage of this current through the resistances of the switching devices and other components that are necessary to drive the LCD causes large amounts of energy to be dissipated.
As a result, hundreds of times the amount of energy that is actually needed to drive each LC element is often wasted because of the large capacitance that is associated with the lines through which the voltages to the elements are delivered.
This large wasted energy is particularly problematic in applications in which energy dissipation needs to be minimized, such as in portable laptop computers. As is well known, the time a single charged battery can run a laptop is a very important specification. The significance of the energy being wasted in driving the lines of an LCD is becoming even more important in view of new technologies that are reducing the energy needed in other areas of the laptop computer. This includes new technologies that are eliminating the need for backlighting of displays and new technologies that are reducing the energy consumed by the microprocessor circuitry and associated storage devices.